The use of hydrated alumina, alone, or in combination with other additives, to impart fire retardance to flexible polyurethane foams is very well known. See, e.g., U.S. Pat. Nos. 3,262,894, 3,810,851, 3,897,372, 3,909,464, 3,943,077, 4,066,578, 4,092,752 and 4,266,042; "Flexible Polyurethane Foam for High Risk Occupancies", J. F. Szabat et al, presented at the Sixth International Conference On Fire Safety, Menlo Park, Calif., January, 1981; "Alumina Trihydrate As A Flame Retardant For Polyurethane Foams", P. V. Bonsignore, "Advances In Urethane Science And Technology", 1981, Vol. 8, pages 253-262 (also appearing in THE JOURNAL OF CELLULAR PLASTICS, July, 1981, pages 220-225). In general, the hydrated alumina described has average particle sizes in excess of 9 microns (see, e.g., page 260 of the Bonsignore article).
It is generally known that fire retardant polyurethane foams can be manufactured by using various combinations of compounds containing chlorine, bromine, phosphorus and antimony. Illustratively, U.S. Pat. No. 3,738,953 discloses the use of a combination of antimony oxide, a polyhalogenated compound and a highly chlorinated hydrocarbon polymer as a flame retardant composition for polyurethane foams. The use of antimony oxide in combination with halogenated compounds is well documented in the art (see, e.g., U.S. Pat. Nos. 3,075,927; 3,075,928; 3,222,305 and 3,574,149). Similarly, the use of phosphorus containing compounds in combination with halogenated compounds (see, e.g., U.S. Pat. Nos. 3,635,821; 3,817,881 and 3,826,762) and in combination with halogenated compounds and antimony trioxide (see, e.g., U.S. Pat. No. 3,448,046 and JOURNAL OF CELLULAR PLASTICS, January, 1965, pages 186-199) has been well documented.
Recently, combustion modified flexible polyurethane foams have been developed which are based on the use of hydrated alumina of an average particle size ranging from 8.5 to 20 microns in combination with antimony trioxide, decabromodiphenyl oxide, a halogenated phosphate ester, a char former, water and, optionally, an organic blowing agent. The polyols used were those of the type described in U.S. Pat. Nos. 4,042,537 and 4,089,835. While the resultant combustion modified flexible polyurethane foams exhibit excellent flame properties, the smoke density values (while acceptable for many applications) are higher than desired. Accordingly, the present invention relates to a foam having improved smoke density values.